FIG. 3 shows an essential part of a conventional centrifugal spark-advance control device for an ignition distributor which comprises a base plate a which is integrally connected to an input shaft which is not shown in the drawing, a pair of centrifugal weights b which are rotatably supported at their points offset from their gravitational centers by pivot pins c fixedly secured to the base plate a, a pulse generator drive shaft d which is rotatably supported on the free end of the input shaft in a coaxial manner, a timing plate e which has the shape of letter "Z" and is fixedly secured to the pulse generator drive shaft d, a pair of tension coil springs g, h which are engaged across pins j of the timing plate e and the pivot pins c of the centrifugal weights b and bias the timing plate e in the direction to delay the ignition timing. As the rotational speed of the input shaft and the whole assembly increases and the centrifugal weights b turn in clockwise direction about the pins c under the centrifugal force acting thereon, the cam surfaces provided in the centrifugal weights b drive the timing plate e in the direction to advance the ignition timing against the biasing force of the coil springs g, h. One of the coil springs h is engaged to the pin j of the timing plate e with a certain gap i therebetween and is adapted to be effective only when the rotational angle of the timing plate e has increased beyond a certain extent and the gap i is eliminated.
According to this conventional structure, if the rotational speed of the input shaft contains any ripple or oscillating fluctuations, the ripple is directly transmitted to the pulse generator and this could cause errors in ignition timing and impair the efficiency of the engine.
Normally, the input shaft of the distributor is connected to a cam shaft for actuating engine intake and exhaust valves by way of an Oldham coupling or the like, and the centrifugal spark-advance control device is interposed between the input shaft and the pulse generator drive shaft. Since the cam shaft receives a force opposing its rotation when each of the cams provided on the cam shaft opens an exhaust or intake valve of the engine against spring force of the valve spring and a force assisting the rotation thereof when the cam closes the valve under the spring force of the valve spring. Such fluctuations in the load acting upon the cam shaft may cause a ripple in the rotational speed of the cam shaft which in turn causes an error in the ignition timing by changing the contact state of the Oldham coupling.
To the end of eliminating this problem, it has been proposed in Japanese Patent Publication No. 59-128978 to provided a cam in the input shaft of an ignition distributor and a device serving as a load to this cam such as a fuel pump is provided in the housing of the ignition distributor in such a manner that generation of vibrations due to the existence of gaps in the Oldham coupling can be prevented by applying a biasing force to the input shaft and closing the gaps at all times.
However, according to this proposal, the addition of a cam and a device serving as a load to the ignition distributor necessitates the increase in the external diameter of the ignition distributor and the length of the input shaft and this is highly disadvantageous in modern automotive engines which are required to be highly compact.
Japanese Patent Publication No. 59-12870 proposes the use of a pair of certain stoppers which are intended to stabilize the angular positions of centrifugal weights but these stoppers are not capable of reducing the vibration of the centrifugal weights or the timing plate.
In view of such problems of the prior art, a primary object of the present invention is to provide a centrifugal spark-advance control device for an internal combustion engine which is free from the fluctuation in the ignition timing due to the twisting oscillation or the ripple in the rotational speed of the input shaft to the ignition distributor.
A second object of the present invention is to provide a centrifugal spark-advance control device for an internal combustion engine which is free from the fluctuation in the ignition timing and is yet compact.
This and other objects of the present invention can be accomplished by providing a centrifugal spark-advance control device for an ignition system of an internal combustion engine, comprising: an input shaft which rotates at a speed corresponding to the rotational speed of the engine; a base plate fixedly attached to the input shaft; a timing member which is rotatably supported on the base plate; a biasing means for biasing the timing member in a certain direction; a centrifugal weight which is pivoted to the base plate at a point spaced from the center of gravity of the centrifugal weight and which is provided with a cam surface for rotatably driving the timing member in an opposite direction against the biasing force of the biasing means by cooperating with a corresponding cam follower surface provided in the timing member; a spark generating means coupled to the timing member for generating ignition sparks in synchronism with the rotational angle of the timing member; a mass member attached to the timing plate by way of an elastic member.
According to this structure, as the mass member serving as an anti-vibration mass which is supported by the elastic member vibrates at a certain phase relationship with respect to the timing plate, the twisting vibration of the timing plate is canceled by the inertia force of the anti-vibration mass and the fluctuations in the ignition timing can be eliminated.
According to a certain aspect of the present invention, the timing member comprising a timing plate which is adapted to rotate in a major plane thereof and the mass member comprises a mass plate which is attached to a major surface of the timing plate by way of an elastic member consisting of a layer of elastomer material. Preferably, the timing plate has a pair of arms extending radially on either side of the rotational center thereof and each carrying the mass plate thereon. These features are helpful for accomplishing a compactness in spark distributor design.
According to another aspect of the present invention, the free ends of the arms of the timing plate are each provided with an extension extending in circumferential direction and the mass plates are carried by these circumferential extensions. This feature enhances the action of the dynamic damper by increasing the moment of inertia of the mass plates.
According to yet another aspect of the present invention, the timing plate and the mass plate are provided with holes which are filled with part of the material of the elastomer layer. Additionally or alternatively, a pin which is integrally connected to the timing plate is passed through the mass plate defining a certain clearance therebetween and this clearance may be filled with the elastomer material in a similar manner. These features improve the durability and reliability of the bonding between the timing plate and the mass plates.
According to yet another aspect of the present invention, the centrifugal weight is provided with a cam follower pin while the timing plate is provided with a cam slot receiving the cam follower pin. Preferably, the mass plate covers a substantially whole major surface of the timing plate except for the cam slot. These features maximize the moment of inertia of the mass plate without impairing the economy of space.